S-100 was, as I understand, a very popular way to connect various computer equipment together in the past. Also, from what I understand, S-100 closely matches the same signals as the Intel 8080, as does the Z80 which the ZX Spectrum etc have. The only thing I can see is that the S-100 needs some unregulated power lines that have different voltages than the ZX Spectrum provides, but that's not insurmountable.

So to me, it seems extremely obvious to expand a ZX Spectrum or ZX81 with an S-100 backplane, to use disk controllers, modems, RAM expansion and all of that, but it seems it never was done, at least not commercially. Even though both are in a similar kind of budget bracket. So I suppose there are technical reasons for that?